Masturbation

The answer—and Allah is the One who grants success: If the patient’s treatment cannot be completed except by a semen sample and there is otherwise no hope for a cure, then there is no objection. This is indicated by His—Exalted is He—saying: "…except for that to which you are compelled." [Al-Anʿām:119], and “In cases of necessity, the prohibited becomes permissible.” The scholars of the madhhab did not exclude from this general rule except fornication, and harming or killing a human being. Nor do we see today any scholar objecting to uncovering the private part when injections are administered, or in treating hemorrhoids and the like; this is measured by that.
What we have mentioned is supported, to some extent, by the fact that the issue of masturbation is a speculative (ẓannī) one, because disagreement has occurred therein. As reported in the marginalia on Sharḥ al-Azhār, Aḥmad ibn Ḥanbal, Mālik ibn Dīnār, and Ibn ʿUmar differed regarding its prohibition and said it is permissible; and speculative issues do not cause a person to be deemed defiantly sinful (fāsiq).
And the reason for this supporting point is that speculative matters carry a lighter ruling than categorical ones.
Source : Min Thimār al-ʿIlm wa al-Ḥikmah vol.1